1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing various types of processing to sheets, such as sorting sheets, and binding the sheets for bookbinding. In particular, the present invention relates to the structure of a sheet conveying portion for conveying the sheets to a sheet processing portion.
2. Related Background Art
Conventionally, examples of a sheet processing apparatus include one structured so as to be capable of implementing processes such as sorting and bookbinding with respect to sheets on which images are formed by an image forming portion for forming images by electrophotography or the like, at high speed and in large quantities. An example of this type of sheet processing apparatus is the one disclosed in Japanese Patent Application Laid-open No. 08-282902.
FIG. 9 is a diagram showing the structure of this type of sheet processing apparatus. In the same figure, reference symbol 1A denotes an image forming portion provided with a photosensitive belt 98, a developing apparatus 104, and the like, reference numeral 4 denotes a sheet feeding apparatus disposed in parallel with the image forming portion 1A and provided with plural sheet receiving portions 2 (2a to 2e) and a separating portion 3, and reference numeral 5 denotes a sheet processing portion that performs sheet sorting, bookbinding, and the like.
In a sheet processing apparatus having this type of structure, a large volume of image forming jobs can be performed at one time by setting sheets used for bookbinding in each of the sheet receiving portions 2 (2a to 2e). It thus becomes possible to perform economy bookbinding operations in large volume and at high speed.
As shown in the same figure, paths P1 to P4 for conveying sheets to the image forming portion 1A are provided between the image forming portion 1A and four sheet receiving portions 2a, 2b, 2c, and 2e, respectively. Sheet types necessary for performing bookbinding of one book can thus be received respectively and separately in each of the sheet receiving portions 2a, 2b, 2c, and 2e. 
As a result, for cases in which thick paper is used for a front cover and a back cover, or when colored paper or tabbed paper is used to sort the contents, for example, bookbinding of a sheet bundle in which the front cover and back cover are thick and the contents are sorted with colored paper or tabbed paper can be performed provided that plain paper, thick paper, colored paper, and tabbed paper are received in the sheet receiving portions 2a, 2b, 2c, and 2e, respectively, the sheets to be used are selected in advance, and the sheets are conveyed in order to the image forming portion 1A.
Further, for cases in which a large volume of identical types of sheets are used, provided that the sheets which are to be used in large quantity are set in plural sheet receiving portions, then even if the sheets in one of the sheet receiving portions are all gone, feeding continues from other of the sheet receiving portions, and the sheets can be replenished during that period. Thus, bookbinding operations can be performed continuously without stopping the system.
Further, with the example shown in FIG. 9, paths P5 and P6 are provided in the two sheet receiving portions 2c and 2d, through which the sheets are conveyed to the sheet processing portion 5 without passing through the image forming portion 1A. Thus, the sheets that are received in the two sheet receiving portions 2c and 2d can be conveyed to the sheet processing portion 5 through the paths P5 and P6 without passing through the image forming portion 1A. Note that the sheet receiving portions 2c and 2d, in which the sheets that do not require image formation are thus received, are hereinafter also referred to as inserters.
For cases in which, for example, the image forming portion 1A is an apparatus dedicated to black and white printing, and one wants to insert color printed sheets in between sheet bundles, or for cases in which the sheets are special sheets that are weak with respect to heat and cannot be made to pass through the image forming portion 1A, the sheets flow in confluence to a path P7 provided between the image forming portion 1A and the sheet processing portion 5, and are conveyed to the sheet processing portion 5 as they are.
Note that a center bookbinding machine provided with a stapler which performs processes from stapling to folding online, a threadless bookbinding machine that glues a back cover onto an aligned sheet bundle, an offset stacker that performs only sorting and aligning and in which bookbinding operations are performed offline, and the like exist as the sheet processing portion 5 provided in this type of sheet processing apparatus. These are used in accordance with the type of processing.
However, conventional sheet processing apparatuses capable of performing these types of economy bookbinding operations in large quantity and at high speed are used in a variety of manners. Desires regarding the type and quantity of the sheets used, the manner of processing, the loading capacity, and the like differ among the users.
For example, in the sheet processing apparatus shown in FIG. 9, the number of the sheet types that can be subjected to bookbinding is limited to four. When a greater number of sheet types are to be used, work becomes necessary for dividing a job into several divisions, and collating them again by using a collator or the like after output. Further, along with increases in color printing speeds, it has been becoming more common recently to insert color printed sheets even in economy bookbinding, and there is also a demand for increasing the number of stages of the inserter for multi-stage construction.
However, it is difficult to perform extension of only the sheet feeding apparatus 4 in a sheet processing apparatus like that of FIG. 9, and it is difficult to respond to the aforementioned demands. Further, it is also difficult to perform extension or exchange of the sheet processing portion according to the load capacity, type of processing, and the like. Therefore, the expandability of the conventional sheet processing apparatus is poor.